Downward flow reactors are used by the chemical and refining industries in a variety of processes, such as hydrotreating, hydrofinishing and hydrocracking. A typical downward flow reactor has a cylindrical outer wall with a catalyst bed positioned within the reactor. The catalyst bed generally rests on a catalyst support grid positioned horizontally within the reactor and spanning the complete diameter of the reactor. The catalyst support grid, together with the outer wall, cooperate to retain the catalyst or other particulate material in place. A distribution tray is positioned horizontally within the reactor at a location above the catalyst bed for evenly distributing process fluids onto the catalyst. The catalyst support grid, outer reactor wall and the distribution tray define the volume of the catalyst bed.
Multiple bed reactors are commonly used. They are formed by providing two or more such catalyst beds spaced along the longitudinal axis of the reactor. The region between successive catalyst beds defines an interbed mixing zone. When a reactor having more than one catalyst bed is used, reactant fluids are introduced into the reactor above the uppermost catalyst bed. The reactant fluids, which typically consist of both liquid and vapor phases, flow through the uppermost catalyst bed.
From the uppermost catalyst bed, unreacted reactant fluids and the related fluid products derived from interaction with the catalyst enter the interbed mixing zone. The interbed mixing zone typically includes a mixing chamber. This interbed mixing zone including a mixing chamber serves several purposes. First, the interbed mixing zone serves as a convenient place through which additional reactants and/or temperature quenching materials can be introduced into the fluid products. In the reactor units described above, heat is released as a result of the reactions between gas and liquid components occurring on the catalyst(s), causing temperature to increase with distance down the bed. In many cases, cool hydrogen-rich gas is introduced between the beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. Secondly, the interbed mixing zone provides a region for mixing the fluid products. Mixing the fluid products prior to reaction in lower catalyst beds ensures more uniform and efficient reactions. In addition, where catalytic reactions are exothermic and temperature control is a critical processing and safety element, mixing of the fluid products within the mixing chamber can be used to eliminate regions of locally high temperature within the fluid products.
The introduction and mixing of quench into the process gas and liquid must be carried out in the interbed space which spans the full vessel diameter, but is often shorter than one vessel radius. Support beams, piping and other obstructions also occupy the interbed region so that unique hardware is required to perform efficient two-phase mixing in what amounts to limited volume.
Poor quench zone performance manifests itself in two ways. First, the quench zone fails to erase lateral temperature differences at the outlet of the preceding bed or, in the worst cases, amplifies them. An effective quench zone should be able to accept process fluids with 50 to 75 degree F. lateral temperature differences or higher and homogenize them sufficiently that differences do not exceed 5 degree F. at the following bed inlet. The second sign of poor performance is that inlet temperature differences following the quench zone increase as the rate of quench gas is raised. This indicates inadequate mixing of cooler gas with the hot process fluids.
Poor quench zone performance limits reactor operation in various ways. When interbed mixing is unable to erase temperature differences, these persist or grow as the process fluids move down the reactor. Hot spots in any bed lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at higher temperatures; hot regions can cause color, viscosity and other qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850 degree F.), the exothermic reactions may become self-accelerating leading to a runaway which can damage the catalyst, the vessel, or downstream equipment. Cognizant of these hazards, refiners operating with poor internal hardware must sacrifice yield or throughput to avoid these temperature limitations. With present day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum quench zone design is a valuable low-cost debottleneck.
One important aspect of the overall mixing efficiency of a quench zone is the ability of the system to mix quench fluids with process fluids. The most critical component of quench mixing efficiency is the methodology though which quench fluid is introduced into the system. There have been various improvements in connection with both physical means and operational considerations for introducing quench into the system.
For example, U.S. Pat. No. 5,152,967 discloses a system incorporating an annular mixing box in which rotational flow of the process fluids is created by slotted entrances. Quench fluid is introduced through an annular ring located substantially in the center of the vessel. The ring is fitted with nozzles to direct quench fluid outward along radial paths. Another device, disclosed in U.S. Pat. No. 5,462,719, creates a rotational flow within a mixing box but without significant liquid holdup in the mixing volume. The quench fluid in this design is introduced through a single vertical inlet at the vessel center positioned such that the entering quench impacts a horizontal deflector forcing the quench fluid radially outward.
Other patents which include descriptions of quench introduction techniques include U.S. Pat. No. 5,635,145. In this patent, a swirl device is used to mix gas and several guide channels are used to mix liquid before depositing them on a pre-distribution tray located between the collection tray and the final distributor tray. Quench is introduced through an annular ring located near the outer wall of the vessel with multiple nozzles directing the flow radially inward. Further, in U.S. Pat. No 5,690,896, an interbed mixing system is described in which an annular mixing trough is used to collect and mix liquid on the collection tray. Gas mixing and further liquid mixing are accomplished in a centrally located mixing box in which the fluids flow in a spiral path towards a central opening in the collection tray. Quench is introduced within the liquid phase through two radially outward oriented nozzles located in the annular mixing trough.
While the above mentioned systems may provide significant improvements in process fluid mixing efficiency, they do suffer from less than ideal quench mixing efficiency. For example, the '967 patent and the '719 patent described above both restrict the process flow through at least one opening passing through the collection tray wherein such openings are substantially in the center of the vessel, and also introduce quench in the space above the center of the tray. As a result, in these designs, quench is injected into a region of the vessel where very high transverse velocities are anticipated. As will be discussed below with respect to the present invention, this configuration results in relatively less efficient quench gas mixing.
The '145 patent also forces flow through a central opening in the collection tray, but locates the quench injection means substantially near the wall of the vessel with multiple nozzles directing quench radially inward. In this design, quench may be suitably located in low-velocity region as it is with the present invention, however, the quench fluid does not enter the vessel flowing counter to the transverse velocity of the process fluid and, as such, does not promote mixing between the process and quench fluids. In this design, it is also believed that the quench location may be located too closely to the wall of the vessel, leaving inadequate mixing volume between the injection point and the wall.
The '896 patent again forces flow through the central opening in the collection tray, but flow is forced to follow a roughly spiral path to the opening as a result of baffles placed on the tray. Quench is injected in two locations on the tray separated by 180 degrees. It is believed that at most one of the quench injectors in this design lies within a low-velocity region beneficial to mixing efficiency. Further, the quench injectors in this system direct fluid radially outward, perpendicular to the transverse velocity of the process fluid rather than opposite the transverse velocity of the process fluid.
Another system, disclosed in U.S. Pat. No. 6,180,068, also provides enhanced mixing of quench gas and process fluids within the interbed space. This system employs separate mixing zones for each of two reactants permitting flexibility in mixing conditions while minimizing pressure drop as well as space and volume requirements. However, the efficiency of this device is sensitive to the degree of phase segregation achieved at the interbed inlet and thus may not perform as desired under all conditions and with respect to particular reactant characteristics.
The above and other known mixing systems generally suffer from the fact that there is insufficient space within the mixing chamber to promote intense two-phase mixing. This limits the capability of these systems to homogenize quench fluid with process fluids and to erase temperature differences between fluids from different sections of the reactor. Accordingly, there is a continued need to provide mixing systems that promote intense two-phase mixing. A preferred system also should provide sufficient volume for the vapor phase to mix separately from the liquid phase. Even while satisfying the above criteria, it is preferable that the designated mixing system minimizes the pressure drop within the reactor as well as permitting relatively easy retrofit with existing reactor spatial constraints.
As can generally be surmised from the above discussion, there is a deficiency in the prior art with respect to efficient mixing of quench fluid with process fluids.